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James Hansen, Pushker Kharecha: Seeing the Forest for the Trees
https://open.substack.com/pub/jimehansen/p/seeing-the-forest-for-the-treesSeeing the Forest for the Trees
James Hansen, Pushker Kharecha
6 August 2025
Abstract. Climate sensitivity is substantially higher than IPCC’s best estimate (3°C for doubled CO₂ ), a conclusion we reach with greater than 99 percent confidence. We also show that global climate forcing by aerosols became stronger (increasingly negative) during 1970-2005, unlike IPCC’s best estimate of aerosol forcing. High confidence in these conclusions is based on a broad analysis approach. IPCC’s underestimates of climate sensitivity and aerosol cooling follow from their disproportionate emphasis on global climate modeling, an approach that will not yield timely, reliable, policy advice.
…
4. Summary: seeing the forest for the trees
Climate change depends on climate sensitivity and the strength of the forcing that drives change. Of the main sources of information – paleoclimate, modern observations, and GCMs – the first two are least ambiguous, but all three are consistent with climate sensitivity 4.5°C ± 1°C (2σ, 95% confidence) for doubled CO₂, which excludes IPCC’s best estimate of climate sensitivity (3°C for doubled CO₂ ). IPCC also underestimates the strength of the aerosol climate forcing.
In the real world, climate sensitivity and aerosol forcing are independent, but they are joined at the hip in climate assessments that focus on the ability of GCMs to reproduce observed global warming. It is reasonable that climate modelers use observed global temperature change to help constrain the GCMs. The complication is that there are two major unknowns: climate sensitivity (mainly because the cloud feedback is uncertain) and the climate forcing (because the aerosol forcing is unmeasured), while there is only one hard constraint (the observed global warming rate). As a result, if climate sensitivity turns out to be high, greater aerosol forcing (i.e., greater aerosol cooling) is required for agreement with observed global temperature.
Independent sources of information, from paleoclimate on climate sensitivity and from satellite data on the cloud feedback, show that, in reality, climate sensitivity is high. Thus, aerosol forcing (and the aerosol cooling effect) have also been underestimated by IPCC. In addition, aerosol cooling has weakened since 2005, mainly because of reduced emissions from China and ships.
…
https://www.columbia.edu/~jeh1/mailings/2025/ForestTrees.06August2025.pdfJames Hansen, Pushker Kharecha
6 August 2025
Abstract. Climate sensitivity is substantially higher than IPCC’s best estimate (3°C for doubled CO₂ ), a conclusion we reach with greater than 99 percent confidence. We also show that global climate forcing by aerosols became stronger (increasingly negative) during 1970-2005, unlike IPCC’s best estimate of aerosol forcing. High confidence in these conclusions is based on a broad analysis approach. IPCC’s underestimates of climate sensitivity and aerosol cooling follow from their disproportionate emphasis on global climate modeling, an approach that will not yield timely, reliable, policy advice.
…
4. Summary: seeing the forest for the trees
Climate change depends on climate sensitivity and the strength of the forcing that drives change. Of the main sources of information – paleoclimate, modern observations, and GCMs – the first two are least ambiguous, but all three are consistent with climate sensitivity 4.5°C ± 1°C (2σ, 95% confidence) for doubled CO₂, which excludes IPCC’s best estimate of climate sensitivity (3°C for doubled CO₂ ). IPCC also underestimates the strength of the aerosol climate forcing.
In the real world, climate sensitivity and aerosol forcing are independent, but they are joined at the hip in climate assessments that focus on the ability of GCMs to reproduce observed global warming. It is reasonable that climate modelers use observed global temperature change to help constrain the GCMs. The complication is that there are two major unknowns: climate sensitivity (mainly because the cloud feedback is uncertain) and the climate forcing (because the aerosol forcing is unmeasured), while there is only one hard constraint (the observed global warming rate). As a result, if climate sensitivity turns out to be high, greater aerosol forcing (i.e., greater aerosol cooling) is required for agreement with observed global temperature.
Independent sources of information, from paleoclimate on climate sensitivity and from satellite data on the cloud feedback, show that, in reality, climate sensitivity is high. Thus, aerosol forcing (and the aerosol cooling effect) have also been underestimated by IPCC. In addition, aerosol cooling has weakened since 2005, mainly because of reduced emissions from China and ships.
…